Exhaust gas recirculation systems (that may be called "EGR systems") are known which are designed to reduce nitrogen oxides (NOx) as harmful components contained in exhaust gases emitted from vehicle engines, such as diesel engines of trucks. The EGR system is adapted to mix a part of exhaust gas of the engine with the intake air of the same engine so as to restrict or lower the combustion temperature and pressure. The exhaust gas recirculated by the EGR system may also be called "EGR gas".
In the engine equipped with the EGR system, the recirculation of high-temperature exhaust gas into the intake air results in an increase in the intake air temperature and a reduction in the volume efficiency, and the engine performance, such as an engine output and fuel economy, may deteriorate. In some cases, the recirculating exhaust gas affects the combustion of an air-fuel mixture, and causes such problems as an increase in other harmful components, including black smoke, in the exhaust gas.
In view of the above problems, various types of recirculating exhaust gas cooling devices (that may be called "EGR cooler") have been proposed and used in practice, for cooling the EGR gas to lower the intake air temperature, for an accordingly improved volume efficiency, so as to improve the engine output, fuel economy and the quality of the exhaust gas.
In the conventional EGR coolers, plate-fin type and multipipe type cooling devices having substantially the same structure as radiators for cooling a coolant of the engine are widely used. In this type of cooling device, however, a large pressure loss occurs when the exhaust gas passes through the EGR cooler, thus making it necessary to increase the volume and weight of the EGR cooler so as to supply a required amount of cooled EGR gas to the engine.
Where the amount of the recirculating EGR gas is to be further increased to reduce a larger amount of NOx in the exhaust gas, and, in particular, where the EGR gas flows into the intake passage of an engine equipped with a supercharger having a high intake air pressure, passages formed through a heat exchanger (core portion) of the plate-fin type or multipipe type EGR need to have an increased cross-sectional area, in order to reduce the pressure loss of the exhaust gas passing through the EGR cooler, and increase the flow rate of the exhaust gas. With the increase in the cross-sectional area of the passages, the volume of the EGR cooler is accordingly increased, resulting in an increased weight of the cooler, and a difficulty in installation of the cooler on the vehicle. In addition, the plate fin type or multipipe type EGR cooler suffers from deposition of unburned substances of the fuel on its pipe walls, since the EGR gas always flows through the cooler only in one direction. Consequently, the cross-sectional area of the passages of the pipes is reduced with the lapse of time in use, and the heat exchanging capability, or cooling capability, deteriorates due to an increase in the pressure loss.
In the meantime, rotary heat exchangers as disclosed in Japanese Laid-open Utility Model Publication No. 2-14570 and Japanese Laid-open Patent Publication No. 7-31718 are known as devices for heating the intake air utilizing high-temperature exhaust gas in gas turbine engines. The rotary heat exchanger includes a heat-exchange core member that is rotatably disposed in a housing in which an intake air passage and an exhaust air passage are located in parallel with each other and next to each other. The heat-exchange core member is formed with a multiplicity of passages that extend in substantially parallel with the rotational axis of the core member, such that these passages communicate with both of the exhaust gas passage and the intake gas passage. This type of rotary heat exchanger, however, has not been used for the EGR cooler.